Laser based frequency standards and their applications

ABSTRACT

Frequency standards based on mode-locked fiber lasers, fiber amplifiers and fiber-based ultra-broad bandwidth light sources, and applications of the same.

CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.11/546,998 filed Oct. 13, 2006, which claims benefit of ProvisionalApplication No. 60/726,617, filed Oct. 17, 2005. The above-notedapplications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to frequency standards based on mode-locked fiberlasers, fiber amplifiers and fiber-based ultra-broad bandwidth lightsources, and applications of the same.

2. Description of the Prior Art

Mode-locked fiber lasers have several advantages over both mode-lockedbulk solid state lasers and mode-locked diode lasers. Mode-locked fiberlasers offer typically superior noise properties compared to mode-lockeddiode lasers and can be packaged in smaller spaces than mode-locked bulksolid state lasers. Mode-locked fiber lasers can be produced withexcellent thermal and mechanical stability. Passively mode-locked fiberlasers in particular can be constructed with few and inexpensive opticalcomponents, suitable for mass production, as disclosed in patentpublication US 2004/0213302 to Fermann et al.

The dispersion compensated fiber lasers as disclosed by Fermann et al.allow the construction of low noise frequency comb sources. Theincorporation of highly nonlinear Bi-fibers was further suggested tominimize the noise of such sources. In addition Fermann et al. disclosedthe design of fiber lasers operating at repetition rates in excess of 1GHz.

Low-noise operation of fiber lasers minimizes their timing jitter,allowing optimized control of the timing of the pulses. In contrast,other prior art modelocked fiber lasers are either limited in repetitionrate or achievable timing jitter.

Generally, the noise background in fiber lasers can be minimized byoperating well above threshold. Hence, modelocked fiber lasers enablingthe oscillation of high energy pulses are very beneficial. Such highpulse energy fiber lasers were previously described in U.S. applicationSer. No. 11/109,711 to Fermann et al. based on the oscillation of nearparabolic pulses inside a positive dispersion fiber laser cavity. Anoptimum in pulse stability is then obtained by the incorporation ofbandpass filters into the cavity. As mentioned in U.S. application Ser.No. 11/109,711 the bandpass filtering action can also come from the gainmedium. The benefits of using parabolic pulses inside modelocked fiberlasers were later reiterated in US patent application 2005/0169324 toIlday et al. The lack of any bandpass filters in these systems restrictsthe operation of such systems to parabolic fiber lasers operating closeto the zero dispersion point and can lead to pulse stability problems.

The '302 publication constituted the first low noise fiber-basedfrequency comb source. Here low noise operation was obtained bycontrolling the fiber cavity dispersion in a certain well-defined range.Low noise operation of fiber frequency comb sources is generallyrequired in order to enable stable locking of the carrier envelopeoffset frequency f_(ceo) of the laser. For example, early work on fiberfrequency comb lasers as disclosed in F. Tauser, A. Leitenstorfer, andW. Zinth, “Amplified femtosecond pulses from an Er:fiber system:Nonlinear pulse shortening and self-referencing detection of thecarrier-envelope phase evolution,” Opt. Express 11, 594-600 (2003) didnot allow locking of f_(ceo) of the laser. Other related work by Tauseret al. in US patent publication 2004/0190119 also does not teach amethod of producing a low noise frequency comb source.

SUMMARY OF THE INVENTION

Compact frequency comb lasers are constructed using ultrafast lasersources in conjunction with highly nonlinear waveguides, such asBi-fibers, silicon waveguides and periodically poled lithium niobate(PPLN) as examples. Specific nonlinear gratings written into suchwaveguides can produce enhanced power in certain narrowband spectralregions within a broadband spectrum, which is highly beneficial for theuse of these devices in metrology applications. The use of nonlinearwaveguides allows the operation of frequency comb lasers at high pulserepetition frequencies (PRF) with reduced power requirements.

Increasing the repetition rate of a modelocked oscillator, results in anincrease in the power in each tooth inside the frequency comb, which hassignificant advantages in actual applications of such combs. Forexample, in optical frequency metrology this leads to higher energyradio frequency beat signals when beating the frequency comb with anunknown optical signal. Moreover, increasing the pulse repetition rateof a mode-locked oscillator also increases the optical mode spacing.This is an important benefit for frequency comb applications asexplained below.

In frequency comb based optical frequency metrology, the frequency ofthe light source to be measured has either to be known with accuracybetter than the mode spacing of the frequency comb used or themeasurement has to be taken at multiple PRFs as disclosed in L-S. Ma etal “A New Method to Determine the Absolute Mode Number of a Mode-LockedFemtosecond-Laser Comb Used for Absolute Optical Frequency Measurements”IEEE Journal of Selected Topics in Quantum Electronics, Vol. 9 p 1066(2003). In both cases a PRF (or equivalently a mode spacing) of largerthan 100 MHz is advantageous: for the first method, wave-metertechnology as typically used to predetermine the wavelength of theunknown source is not readily available with a resolution better than100 MHz. For the second method the requirements of the necessary PRFdetuning to be able to determine the mode number is relaxed at higherPRF.

The phase noise of the frequency comb source can be further reduced byusing highly nonlinear bismuth oxide based fibers with positivedispersion for supercontinuum generation. In such a system Ramansolitons which are known to have large phase noise are suppressed.

For the application of photonic analog-to-digital conversion of highspeed electrical signals a high PRF laser source with low timing jitteris necessary which can be provided by this invention.

A high PRF laser source with low timing jitter has also advantages forfemtosecond laser optical comb based distance metrology when using selfbeat down technology as disclosed by Minoshima et al. in “Study oncyclic errors in a distance measurement using a frequency comb generatedby a mode-locked laser”, Conference on Lasers and Electro-optics 2004,paper CTuH6. To enhance the distance resolution in femtosecond laserfrequency comb based distance metrology a high harmonic of the PRF attypically 10 GHz is used for detection. To minimize cyclic error,adjacent harmonics to the harmonic used for detection need to besuppressed by more than 20 dB. A high PRF laser source relaxes therequirement for the bandwidth of the RF filter used in the detectionsystem. A suitable laser source can be provided by this invention.

Low phase noise, high PRF fiber systems also allow the construction offiber lasers in the whole telecom band in the 1.50-1.65 μm wavelengthregion and their application to optical sampling as well as A/Dconversion.

Other applications of low phase noise, high PRF fiber systems comprisewave meters, optical clocks, and sources of pulse trains with absolutephase stability. Low phase noise, high PRF fiber systems can further beeffectively applied for the comparison of frequency standards atdifferent locations or the transport of frequency standards along fibertransmission lines. To minimize the loss of frequency precision alongfiber transmission lines, schemes for length control of the fibertransmission lines as well as signal repeaters using local oscillatorscan be implemented.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 a) is a diagram of a generic spectral supercontinuum sourceaccording to an embodiment of the invention.

FIG. 1 b) is a diagram of a highly nonlinear waveguide incorporatingseveral frequency conversion sections according to an embodiment of theinvention.

FIG. 1 c) is a representative plot of a supercontinuum generated in aPPLN waveguide.

FIG. 1 d) is a representative plot of an RF spectrum showing f_(ceo)beat signals within the supercontinuum generated in a PPLN waveguidewhen using a modelocked fiber laser.

FIG. 2) is a diagram of a carrier envelope phase-locked frequency combsource according to an embodiment of the invention.

FIG. 3) is a diagram of an optical wave meter according to an embodimentof the invention.

FIG. 4) is a diagram of an optical clock according to an embodiment ofthe invention.

FIG. 5) is a diagram of a phase-stable fiber laser according to anembodiment of the invention.

FIG. 6) is a diagram of a fiber based analogue/digital converteraccording to an embodiment of the invention.

FIG. 7 a) is a diagram of a fiber system for thetransmission/distribution of frequency standards along fibertransmission lines.

FIG. 7 b) is a diagram of an optical path length-stabilized fiber link.

FIG. 7 c) is a diagram of a signal recovery repeater for an opticalfrequency comb.

FIG. 8) is a diagram of a fiber optical sampling unit according to anembodiment of the invention.

FIG. 9 a) is a diagram of a fiber frequency comb which is passivelyenhanced in pulse energy by an external cavity.

FIG. 9 b) is a diagram of an amplified fiber frequency comb withmultiple parallel amplifiers which are coherently combined.

DETAILED DESCRIPTIONS OF SPECIFIC EMBODIMENTS

For most applications of frequency combs, low noise operation offrequency comb lasers is preferred. Low noise dispersion compensatedfrequency comb sources are described in Fermann et al. '302, which isherein incorporated by reference. Applications of frequency comb sourcesas described in Fermann '302 greatly benefit from the availability offrequency combs that actually span more than one octave in the opticalfrequency spectrum.

The generic design for an octave spanning comb source is shown in FIG. 1a. Such an octave spanning source typically comprises a modelocked fiberoscillator, whose output is further amplified in additional fiberamplifiers, where the amplified output is then injected into highlynonlinear fibers. An octave spanning spectrum typically is achieved oncethe output from the oscillator reaches the highly nonlinear fiber.

Here, oscillator 120 represents an oscillator design as previouslydescribed in the '302 publication. However, any other oscillator design,including modelocked bulk solid-state laser oscillators, modelockeddiode lasers or mode-locked waveguide lasers may also be implemented.The oscillator output is amplified in amplifier 121, which can beconstructed from a fiber or, more generally, a waveguide amplifier. Forhigh power systems, the amplifier 121 can also be omitted. Fiber pigtail122 connects oscillator 120 and amplifier 121. The output from amplifier121 is obtained via pigtail 123 and is coupled into highly nonlinearfiber (or waveguide) 124 via splice 125. It is preferable to minimizethe pulse length at splice 125 to maximize the spectral broadening infiber 124. The pulse length can be minimized by the insertion ofappropriate dispersion compensating fiber segments between theoscillator and amplifier or after the amplifier output, as discussed inthe publication to Fermann et al. Specific design examples are thereforenot further described here. To maximize spectral broadening in segment124 while adding a minimum of noise, preferably a dispersion engineeredholey Bi fiber with slightly positive dispersion is implemented.Dispersion engineering can be implemented by the incorporation ofair-holes into the fiber cladding.

Bismuth oxide based highly nonlinear fibers have been discussed in K.Kikuchi et al. “Highly-nonlinear bismuth oxide-based glass fibers forall-optical signal processing” OFC 2002, post deadline paper. Oneadvantage of the use of Bismuth oxide over silica based fibers is thepositive dispersion of the fiber at around 1.5 um. Spectral broadeningin positive dispersion fiber avoids Raman soliton formation which isknown to introduce excess noise. Excess noise limits the performance offrequency comb sources. Another advantage of highly nonlinear Bismuthoxide fiber is the higher non-linearity compared to standard silicabased fiber, maximizing the amount of spectral broadening.

Dispersion engineering of microstructured Bismuth oxide fiber furtherincreases its nonlinearity. Dispersion engineering can reduce thedispersion of Bi-fibers, maximizing spectral broadening at low pulseenergies as generated with high repetition rate pulse sources.

As discussed in the '302 publication, the use of low noise pump lasersis preferred for frequency comb lasers, since noise of the pump laserleads to excess noise in the modelocked oscillator. This is especiallyimportant for noise at spectral frequencies below the lifetime of theactive laser transition in the modelocked oscillator where no filteringaction of the laser medium is present. Pump lasers ideally operate in asingle longitudinal mode. Pump laser noise may be reduced either byactive noise cancellation via acousto-optical modulators or passively bydiode pumping a cw solid state laser which is used as pump laser for themodelocked fiber oscillator. In the latter scheme the cw-laser acts as alow pass filter and pump diode noise above the lifetime of the lasertransition of the cw-laser is greatly reduced.

The coupling of external pump lasers to fiber lasers and the use ofacousto-optical modulators for noise cancellation is well known in thestate of art and is not further discussed here.

Also any mechanical perturbation of the pump fibers and also theoscillator fibers must be avoided since they can couple to the opticalfrequencies of the system output and lead to excess noise. Ideally allfibers are mounted so as to be acoustically isolated.

As further discussed in the '302 publication, the carrier envelopeoffset frequency f_(ceo) is conveniently controlled by modulation of theoscillator pump power that is injected to the modelocked oscillator. Anyperturbation of the laser action induced by modulation of the oscillatorpump power is low pass filtered by a time constant related to the upperlaser level lifetime of the laser medium used for the oscillator.Therefore to electronically control the carrier envelope offsetfrequency with a high bandwidth, an upper laser level with a shortspontaneous emission life-time is preferred. For the case of an Er fiberlaser, the relevant laser level is the ⁴I_(13/2) transition to theground state. In standard Er-doped silica fiber the ⁴I_(13/2) level hasa spontaneous emission life-time of 10 ms. In contrast in Bi-fiber, aspontaneous life-time of 3 ms can be achieved. Hence the construction offiber comb sources based on modelocked Er Bi-fiber oscillators based onEr-doped Bi-glass allows for the electronic reduction of the noise inthe carrier envelope offset frequency compared to a frequency combsystem based on Er-doped silica fiber.

For the same reason, Yb, Nd or Tm doped silica fiber, with spontaneousemission life-times of 1 ms, 0.5 ms and 0.025 ms respectively allow foreven higher bandwidth control of the carrier envelope offset frequency.

In addition to Bi-fibers or holey Bi-fibers, Bi-waveguides, siliconwaveguides or other highly nonlinear waveguides such as waveguideswritten into LiNbO₃ can be used for spectral broadening. To minimizecoupling losses between segments 123 and 124, a highly nonlinear silicafiber can be used in place of segment 123, while segment 124 can be ahighly nonlinear Bi-fiber.

Highly nonlinear LiNbO₃ waveguides have the further advantage thatnonlinear spectral broadening can be combined with the construction ofan interferometer as required for phase control of passively modelockedlasers. The basic principles of operation of an interferometer forcarrier envelope phase detection were first discussed by H. R. Telle etal. in ‘Carrier-envelope offset phase control : a novel concept forabsolute frequency measurement and ultrashort pulse generation’, Appl.Phys. Lett., B69, 327-332 (1999) and are described also in the '302publication to Fermann et al. The generic design of a highly nonlinearLiNbO₃ waveguide in conjunction with an f-2f interferometer is shown inFIG. 1 b. The LiNbO₃ substrate contains waveguide 131 and a frequencyconversion section 132 based on periodic poling. The light is coupledinto end face 133 and extracted via end face 134. Self-phase modulation,cascaded second-order nonlinearities and other nonlinear processes inwaveguide section 131 produce a greatly broadened optical spectrum. Toenhance the spectral broadening effect, a periodically poled section 135can be implemented close to the endface 133 of the waveguide. The polingperiod and grating length can be optimized for the desired broadeningmechanism. Multiple poling periods or chirped gratings are alsopossible. The implementation of these gratings are state of the art andneed no further discussion. A part of the broadened spectrum A₀(ω) canbe frequency doubled in section 132 to produce a signal at A(2ω). Thefrequency doubled spectral component A(2ω) can then interfere with thenon-converted spectral component A₀(2ω) to produce a beat signal betweenthe doubled and non-converted spectral components. This beat signal canin turn be used to control the absolute phase of the modelockedoscillator upstream of the nonlinear waveguide. Alternatively differencefrequency mixing can also be used in section 132 to produce a beatsignal between frequency-converted and non-converted spectralcomponents. For difference frequency mixing, a high frequency spectralcomponent A₀(2ω) couples with a low frequency component A₀(ω) to producea frequency down-converted signal at A(ω). The beat-signal then resultsfrom interference between the signals A₀(ω) and A(ω).

An octave spanning spectrum is not required when incorporating an 2f-3finterferometer. For example the red part of the continuum can befrequency tripled and interfered with the frequency doubled blue part ofthe continuum. Such implementations are well known from continuumgeneration in highly nonlinear fibers and are not further discussedhere.

Instead of periodic poling, parasitic nonlinearities inside the LiNbO₃waveguide can also be utilized for harmonic conversion of spectralregions of the continuum. To simplify coupling, the waveguide may alsoinclude tapered sections close to the end face. This allowsmode-matching to butt coupled single mode fibers. The mode fielddiameter at the waveguide end-face is matched to the fiber mode fielddiameter and is then adiabatically reduced to enhance nonlinearities.

The spectrum of a supercontinuum generated in a PPLN waveguide is shownin FIG. 1 c. To obtain the supercontinuum, 50 fs pulses from an Eramplifier with a pulse energy of 2.5 nJ were coupled into the waveguide.A broad continuum covering the spectral region from 500 nm to 2500 nm isclearly visible. The supercontinuum spectrum has several narrow spectralspikes in the 1000 nm wavelength region. These spectral spikes arecreated when frequency doubling the continuum near 2000 nm. To obtainthe narrow spectral output in the 1000 nm spectral region a polingperiod of 26.5 μm was implemented. Secondary spikes arise from frequencydoubling to higher order modes which could be eliminated with anall-single mode design. By appropriately designing the poling period ofthe LiNbO₃ waveguide, (i.e. by appropriately designing the PPLN gratingperiod), enhanced spectral density can be obtained at very wellspecified spectral points. Moreover, as shown in FIG. 1 b, severalgrating periods can be written into a PPLN waveguide, thus allowing forthe simultaneous generation of several regions of enhanced spectraldensity. Such regions of high spectral density in the continuum are veryuseful in frequency metrology applications when comparing the continuumoutput to the output of an optical reference signal as explained withregards to FIGS. 3 and 4 below.

A beat signal between the frequency doubled portion of thesupercontinuum near 1400 nm and the frequency tripled portion of thesupercontinuum near 2100 nm was observable at 700 nm. This beat signalcan be related to the evolution of the carrier envelope offset frequencyof the oscillator and can be used for phase locking of the carrierenvelope of the pulses inside the oscillator. An RF spectrum of theoutput near 700 nm measured with a fast photo-diode is shown in FIG. 1d. The beat signals near 205 MHz and 225 MHz arise from evolution of thecarrier envelope phase inside the modelocked laser.

The fiber laser and amplifier systems as discussed above canconveniently be used in low-noise, high repetition rate frequency combsources. FIG. 2 shows a frequency comb source based on a mode-lockedEr-doped bismuth oxide fiber oscillator 201. Such modelocked oscillatorswere previously described in the '302 publication and can be constructedby replacing the active Er silica fiber with the Er bismuth oxide fiber.No further discussion is required at this stage. However, silica basedfiber oscillators based on low-dispersion cavity designs could also beimplemented. Because of the low noise of parabolic pulse oscillators,parabolic pulse oscillators as described in the '711 patent can also beused.

The oscillator output is amplified by the active element 210 whichconsists of Er or Er/Yb doped silicate or Bismuth oxide fiber. Theoscillator, amplifier and the amplified output 219 may be isolated byisolators 202 and 218. Part of the oscillator power may be outputcoupled by a fused fiber coupler to the output 204 which can be used todetect and stabilize the PRF of the oscillator with feedback loops, orfor further additional optical amplifiers. Alternatively the output 220can be used for PRF detection and stabilization. The amplifier isoptically pumped by fiber coupled laser diodes 205,207,213,214 which maybe polarization multiplexed by the polarizing beam combiners 203 and216. The pump light is coupled into the active fiber by WDM couplers 208and 212. An additional fiber 217 can be used for dispersive pulsecompression. Part of the amplified light can be output coupled by afiber coupler or a WDM coupler (not separately shown) to the output 220.

For carrier envelope offset phase slip detection and feedbackstabilization several schemes can be used, as discussed in H. R. Telleet al. “Carrier-envelope offset phase control: A novel concept forabsolute optical frequency measurement and ultrashort pulse generation”Appl. Phys. B 69, 327-332 (1999), and as discussed also in the '302publication. When implementing an f to 2f interferometer for carrierenvelope phase control, an octave spanning spectrum is required, whichis conveniently generated in a highly nonlinear fiber as discussedabove. Highly nonlinear silica based fibers were discussed in T. Okunoet al. “Silica-Based Functional Fibers with Enhanced Nonlinearity andTheir Applications” IEEE J. Sel. Top. Quantum Electron. 5, 1385 (1999).

The f to 2f interferometer is realized by a group delay compensatingfiber 224, a second harmonic generation crystal 226, a delivery fiber228, relay optics 225, 227 and 229 and a photo detector 230.

Part of the optical comb output can be extracted by fusion couplers orbulk optics after the nonlinear fiber 222, after the group delaycompensating fiber 224 or at the delivery fiber 228. The fusion splices206,209,211,215,221 and 223 connect the different optical elements orfibers.

For self referencing carrier envelope offset phase slip stabilizationthe beat signal at the detector 230 is fed back to the oscillator byelectrical control loop 231. The electrical control loop is realized asa phase locked loop which compares the beat signal frequency f_(CEO)with a reference signal. In a preferred embodiment the reference signalis a fraction of a multiple of the oscillator PRF. In another embodimentthe reference signal is an RF oscillator phase locked to a frequencystandard, for example a GPS synchronized Cesium or Rubidium Clock. Phaselocked loops and GPS synchronized Cesium and Rubidium clocks are wellknown and need no further description.

To control the carrier envelope offset frequency, the oscillator pumppower, the fiber Bragg grating temperature or stress and the saturableabsorber temperature can be modified. Such schemes were alreadydiscussed in '302 and are not further described here. The temperature ofa saturable absorber can be controlled by mounting the saturableabsorber on a heat sink and stabilizing the temperature of the heatsink. Such schemes are well known in the state of the art and need nofurther explanation.

For several applications the PRF of the self referenced oscillator needsto be phase locked to a reference oscillator. This is done by a secondcontrol loop. The PRF of the laser can be detected at the detector 230or preferably with a second photodiode at output port 204 or output port220. The control loop uses preferably a high harmonic of the PRF asinput signal and a low phase noise RF oscillator locked to a frequencystandard as reference signal. High harmonics of the PRF can be detecteddue to intermodal beating of the frequency comb. As discussed in D. vonder Linde “Characterization of the noise in continuously operatingmode-locked lasers” Appl. Phys. B. vol. B 39 p. 201-217 (1986)oscillator phase noise is dominant in high harmonics of the PRF.Therefore a stabilization loop which uses high harmonics of theoscillator as input signal and is referenced to a low phase noise RFreference oscillator leads to reduced phase noise of the optical comb.The control signal for the PRF stabilization is fed back to the fiberoscillator. The PRF of the fiber oscillator can be tuned by changing thecavity length using Piezo-electric actuators as discussed in '302.Additionally PRF tuning by stretching or heating of the oscillator fiberis possible as also discussed in '302.

FIG. 3 shows a generic embodiment of an optical wave meter based on aself-referenced optical fiber frequency comb source. PRF and f_(CEO) ofthe optical frequency comb 303 are locked to RF clock 301 which can befor example a GPS synchronized Cesium or Rubidium clock. A cw laser 304whose optical frequency is to be determined is combined with the opticalfrequency comb in a fiber optic combiner 305. The beat signal betweenthe frequency comb and the cw laser is detected by a photodiode 307. Toimprove the signal to noise ratio of the beat signal, an optical bandpass filter 306 which is centered at the wavelength of laser 304 and anelectrical band pass filter 309 which is centered at the beat frequencyare inserted in the optical and electrical signal paths. Thesignal/noise ratio can further be maximized by providing regions ofenhanced spectral intensity overlapping with the spectral output of thecw laser. Such regions of enhanced spectral intensity can be for exampleobtained in the supercontinuum output of appropriately designed highlynonlinear PPLN waveguides, as explained above with regards to FIG. 1 b).

The frequency of the beat signal f_(beat) is counted by a frequencycounter 308 which is referenced to the RF clock 301. The absolutefrequency of laser 304 is determined by f=±f_(CEO)+n*PRF±f_(beat). Theunknown signs of f_(CEO) and f_(beat) as well as the mode number n canbe determined by repeated measurements of f_(beat) at slightly differentPRF and f_(CEO) as described in L-S. Ma et al “A New Method to Determinethe Absolute Mode Number of a Mode-Locked Femtosecond-Laser Comb Usedfor Absolute Optical Frequency Measurements” IEEE Journal of Selectedtopics in quantum electronics, Vol. 9 p 1066 (2003). The mode number ncan be determined alternatively by measuring the frequency of laser 304by a conventional Fabry Perot type wave meter with resolution betterthan the frequency comb spacing which is equal to the oscillator PRF.

As disclosed by Diddams et al., “An Optical Clock Based on a SingleTrapped 199Hg+Ion”, Science 2001 293: 825-828 a self referenced opticalfrequency comb can be used as an optical clock if one comb line is phaselocked to an optical frequency standard. However in this publication aself-referenced optical comb based on a bulk Ti:Sapphire laser was used.The Ti:sapphire laser based comb has several disadvantages such as lowlong term stability, bulkiness, large size and large power consumption.

FIG. 4 shows the embodiment of an optical clock using the selfreferenced fiber laser based optical frequency comb 411. The frequencycomb system 411 was described with respect to FIG. 2. The optical outputof the self referenced fiber laser based frequency comb 411 is combinedby a fused fiber combiner 418 with the output of an optical frequencystandard 412 operating at the frequency f_(OPT). Optical frequencystandards consist typically of a cw laser which is locked to anultra-stable high-Q optical cavity and stabilized to an atomic ormolecular transition and are well known in the field. The PRF of theoscillator and its harmonics generated by intermode beating are detectedby a photodetector 407 at the optical output 408 which can beimplemented by output port 204 or 220 of the frequency comb laserillustrated in FIG. 2. Alternatively one of the photodetectors 422 or425 can be used for PRF detection. The detected signal containing thePRF frequency and its harmonics is an exact fraction of f_(OPT). Part ofthe RF output is used as reference signal 402 for the feedback loops. Abandpass filter 405 can be implemented to select the k-th harmonic ofthe PRF as a reference signal.

With the first feedback loop the carrier envelope offset frequencyf_(CEO) is phase locked to a fraction n of the reference signal usingthe f-2f interferometer 420, the optical bandpass filter 421, thephotodetector 422, the RF bandpass filter 423, the phase detector andloop filter 413 and the divider 403. The phase detector and loop filter413 generates an error signal 414 which is fed back to control the CEOphase slip of the frequency comb laser. With this feedback loop in placef_(CEO) is stabilized to f_(CEO)=±k/n*PRF. The second feedback controlloop is used to phase lock the optical comb line which is closest tof_(OPT) to f_(OPT) plus a small RF offset f_(beat). The beat signalf_(beat) between the frequency comb and the optical frequency standardis detected by a photodiode 425. The second feedback loop is constructedsimilar to the first feedback loop. To improve the signal to noise ratioof the beat signal an optical band pass filter 424 which is centered atthe wavelength of the optical frequency standard 412 and an electricalband pass filter 426 which is centered at the beat frequency areinserted in the optical and electrical signal paths. The phase detectorand loop filter 409 generates the error signal 410 by measuring thephase difference of the beat signal and a fraction m of the referencesignal. The error signal 410 is used to control the PRF of the frequencycomb laser. With both feedback loops in place the PRF of the frequencycomb laser is in fixed relation to f_(OPT) withPRF=f _(OPT)/(l±k/n±k/m)where l is a large integer representing the mode number of the frequencycomb mode closest to f_(OPT). The signs in the above equation can bedetermined by changing n and m. Other embodiments of an optical clockmay include additional RF or optical amplifiers, tracking oscillatorsand other electronic controls. Other embodiments may also includefrequency conversion stages (e.g., second harmonic generation) tooverlap the output spectrum of the frequency comb with the opticalfrequency standard. In another embodiment the optical output 408 of thefrequency comb laser might be also used for optical transmission of timeand frequency information.

As disclosed by J. J. McFerran et al in “Low-noise synthesis ofmicrowave signals from an optical source,” Electron. Lett. 41, 36-37(2005) the above setup can also be used as a low noise RF signal source.For the application as a low noise RF signal source, the long termstability requirements are typically relaxed and the optical frequencystandard can be implemented as a cw laser locked to a stable high-Qcavity. Low noise RF sources are needed in several technological areasincluding navigation, radar, remote sensing, high speed electronics,clock distribution, and communications. Frequency-comb systems used inthose applications need to be compact and rugged to be suitable forportable instrumentation without compromising stability. Theimplementation disclosed in the above publication uses Ti:Sapphire lasertechnology which has the previously described disadvantages and is notcompatible with portable instrumentation. For low phase noise RFgeneration special attention has to be paid to the implementation of thephotodetector 407 and excess noise in the photodetection process has tobe minimized. Here the use of a high PRF Yb oscillator or an Er-dopedbismuth-oxide oscillator has several advantages. For Er oscillators, thecenter wavelength of the oscillator around 1550 nm is at the sensitivitymaximum of low noise InGaAs PIN-photodetectors allowing high extractedRF powers which limit the shot noise related phase fluctuations. Second,since many applications demand stable RF sources around 10 GHz, it isdesirable to extract the highest possible RF power at a harmonic near 10GHz. The total optical power on the photodetector is limited by detectordamage or detector saturation and therefore the total RF power is alsolimited. Therefore the RF power per PRF harmonic can be maximized byhigher PRF which leads to fewer RF harmonics within the detectionbandwidth of the photodetector.

As described above, the excess noise on the generated supercontinuum canbe highly reduced by the use of positively dispersive Bismuth oxidebased highly nonlinear fibers for spectral broadening because of theavoidance of Raman solitons. With such a low noise fiber laser basedfrequency comb, time domain applications of the carrier envelope offsetfrequency stabilized laser are feasible. FIG. 5 shows a modified f-2finterferometer which allows the stabilization of the carrier envelopeoffset phase slip to zero. Zero phase slip is advantageous for timedomain applications since all the pulses emitted from the oscillatorhave the same carrier envelope phase. Stabilization to zero carrierenvelope offset phase slip is not possible with the standard f-2finterferometer described above since the beat note related to thecarrier envelope offset phase slip overlaps with the intermode beatrelated to the PRF. In the modified f-2f interferometer the output 502of the frequency comb laser 501 is split by the fusion splitter 503which is preferably a WDM splitter. The low frequency part of the octavespanning spectrum in output 504 is frequency doubled by a nonlinearcrystal 508 and combined with the high frequency part of the spectrumfrom output 505 which is frequency shifted by a modulator 507 which isdriven by a stable RF frequency 506. The two arms of the interferometerare combined by fusion splitter 509 and the beat frequency is detectedon a photo detector 510. The beat note related to the carrier envelopeoffset phase slip is now frequency shifted by the driving frequency ofthe modulator 506 and can be separated by an RF filter from theintermode beat signals related to the PRF at zero carrier envelopeoffset phase slip. This enables the use of a feedback control loop 511for stabilizing the carrier envelope offset phase slip to zero.

Phase-stabilized pulses can be used to resonantly enhance the pulseenergy in a passive external cavity downstream from the frequency comblaser. In this application, the resonant enhancement cavity has the sameround-trip time as the time separation between the pulses, which areinput to the resonant enhancement cavity. With such resonant enhancementcavities, absolute pulse phase stabilization of the pulses is notrequired, rather it is sufficient to stabilize the comb laser modes tothe cavity such that they spectrally overlap with the cavity resonancesover a significant part of the comb laser spectral envelope, as forexample described by R. Jason Jones et al. in “Precision stabilizationof femtosecond lasers to high-finesse optical cavities”, PHYSICAL REVIEWA 69, 051803(R) (2004). In the scheme by Jones et al., the reflectedsignal from the input coupler into the enhancement cavity can be used aserror signal for two feedback loops to stabilize both PRF and the CEOrelated frequency offset of the femtosecond oscillator/amplifier comb tothe enhancement cavity. Such a stabilization scheme can replace the f-2finterferometer. The stabilization method described in U.S. Pat. No.6,038,055 to Hansch et al. only uses one feedback loop which is notsufficient to control both PRF and CEO phase. Alternatively to stabilizethe frequency comb laser PRF to the cavity mode spacing, the cavity modespacing can be stabilized to the frequency comb PRF for example bytranslating a cavity mirror.

FIG. 9 shows an embodiment of a frequency comb laser stabilized to anenhancement cavity. The frequency comb laser consists of a dispersioncompensated fiber oscillator 901 of similar design to the oscillator 120described above and a fiber amplifier 903. To reduce the non-linearphase shift related to pulse amplification, chirped pulse amplificationis implemented, i.e. a pulse stretcher 902 is used before amplificationand a pulse compressor 904 compresses the pulses after amplification.Fiber chirped pulse amplification systems were previously described inU.S. Pat. No. 6,885,683 and do not need to be further discussed here.

The output of the frequency comb laser is coupled into the enhancementcavity through an input coupling mirror 906. The enhancement cavity isformed by this input coupling mirror and additional high reflectormirrors 908, 912 and 913. For optimum impedance matching, thetransmission of the input coupling mirror is approximately equal to thesum of all other cavity losses. The optical beam path 915 containscombined radiation which is coupled out or reflected by the inputcoupling mirror 906. A spectrally narrow selected part of this radiationcan be used to generate the error feedback signal for example accordingto the method described in “Laser frequency stabilization bypolarization spectroscopy of a reflecting reference cavity” by Hanschand Couillaud, in Optics Communications, Volume 35, Issue 3, p. 441-444(1980). The feedback signal 911 is used to control the oscillator PRFand therefore align the oscillator comb spacing to the cavity combspacing within the above spectrally selected part. For CEO stabilizationeither a second spectral part can be selected and the feedback can beimplemented in a similar way as the PRF feedback, or the leakageradiation through a high reflector mirror which is on averageproportional to the intracavity average power can be detected by a slowphotodetector 910 and used for feedback control for CEO stabilization,where typically only a slow bandwidth feedback loop is required in thisparticular application. Since the cavity modes are only equidistantlyspaced for zero intracavity dispersion, preferably low dispersion ordispersion compensating mirror coatings are used and the cavity isplaced inside an evacuated enclosure 907.

Two of the cavity mirrors can be curved to form an intracavity focuswhich can be used to reach very high field intensities. As disclosed in“Phase-coherent frequency combs in the vacuum ultraviolet viahigh-harmonic generation inside a femtosecond enhancement cavity” PhysRev Lett., 2005. 94(19): p. 193201 by Jones et al. this can be used forthe generation of VUV, EUV and XUV radiation by inserting an inert gasat the intra-cavity focus. Alternatively, another nonlinear material canbe inserted at the intra-cavity focus to induce a nonlinear interactionto generate an output at frequency-shifted wavelengths. For example anonlinear crystal such as PPLN or GaAs can be inserted at the focus forTHz generation. The enhancement cavity can thus greatly increase theefficiency of THz generation compared to schemes where no enhancementcavities are used. Equally a parametric amplifier crystal can beinserted at the intra-cavity focus to enable the amplification of pulsesat different wavelengths from the pump wavelength. Since the enhancementcavity greatly increase the pump signal intensity at the intra-cavityfocus, parametric amplification to greatly enhanced pulse energiescompared to single-pass schemes is enabled. Optionally added nonlinearmaterials such as gases or nonlinear crystals are indicated by dottedoutline in FIG. 9.

The system disclosed by Jones et al has the disadvantage that bulk solidstate oscillators are used which are limited in average powers due tothermal effects. This is greatly reduced by using fiber amplifiersystems.

The amplification of a cw master oscillator by multiple parallel fiberamplifiers and the coherent addition of the amplifiers by feedback phasecontrol was disclosed for example by Augst et al. in “Coherent beamcombining and phase noise measurements of ytterbium fiber amplifiers” inOptics Letters, Vol. 29, Issue 5, pp. 474-476 (2004) and referencestherein. However in the publication by Augst et al. continuous waveradiation was amplified and therefore limited peak powers were reached.

FIG. 9 b shows an embodiment of an amplified frequency comb oscillatorwith multiple parallel amplifiers which are phase-coherently combined.Here, in contrast to Augst et al., short pulses are amplified inparallel and coherently added with the advantage of much higher combinedpulse peak power.

The output of the oscillator 950 is stretched by the stretcher 972 andsplit into multiple parts which are delivered by fibers 951,952 and 953to the optical delay stages 955, 956 and 957 which provide controllablepath length delay (which can be realized by mechanical delay for coarsedelay, and/or acousto optical modulators). The output of the delaystages is amplified in fiber amplifiers 958,959 and 960 which can berepresented as separate amplifier fibers or active cores of a multi-coreamplifier fiber. In this embodiment three delay stages and amplifierstages are shown as an example only, any number of amplifier stages canbe implemented. The outputs of the amplifier fibers are tiled, which canbe either accomplished by arranging the amplifier fibers in an array andcollimating with a lens array or individually collimating the amplifierfibers and tiling the beams with mirror optics as shown in the figure.The tiled beam can be compressed in the compressor 971 and focused by alens 965 onto a target 966. A small fraction of the tiled off beam ispicked off by the partial reflector 964 and combined with a part of theoscillator output which is split off into the fiber 954 to act as areference beam, and is optionally frequency shifted by anacousto-optical modulator 970. The delay between the reference beam andthe amplifier output beams is detected in the phase sensor and pulsedetector array 968. Feedback signals are thus generated for the delaystages 955, 956 and 957 in a way that maxima of the pulse envelopes ofthe individually amplified pulses coincide at the target 966 and theelectrical fields coherently add up. The phase sensor array can beimplemented, for example, by optical cross-correlation measurement, RFheterodyne beat detection or optical interferometry. Also a small cwlaser at a different wavelength can for example be injected via a beamsplitter into stretcher 972. The phase of the cw signal of the output ofthe amplifier array 955-957 can then be stabilized to allow for coherentaddition of the pulsed output of the amplifier array. Such animplementation is not separately shown.

FIG. 6 shows an embodiment using the fiber based frequency comb laser asa clock for a photonic sampler for A/D conversion. Photonic samplers forA/D conversion are discussed in A. S. Bhushan et al. “150 Gsample/swavelength division sampler with time-stretched output,” Electron.Lett., vol. 34, no 5, pp. 474-475, 1998. One of the limiting factors ofthe described system is the timing jitter of the optical sourceproviding the sampling pulses. The above described frequency comb laserhas the advantage of low timing jitter due to a truly dispersioncompensated cavity design. In this exemplary embodiment the spectrum ofthe frequency comb laser 601 is chirped by a dispersive element 602. Toenhance the sampling rate to a factor of n of the oscillator PRF a(n−1)-stage optical delay line 603-605 is used. Such a differentialoptical delay line is current state of the art and needs no furtherdescription.

In another embodiment the differential delay line precedes thedispersive element. The RF signal to be sampled 607 is encoded on theoptical signal by a modulator 606. The optical signal is then split intom multiple wavelength channels by an arrayed waveguide grating 608. Theoptical signal of each channel is detected by a photodetector 609 anddigitized by m standard A/D converters 610 (only two such A/D convertersare shown for simplicity). Since the optical pulses are chirped, the RFsignal is encoded on different wavelengths at different times. Thereforethe channels of the AWG grating provide a time interleaved sampling ofthe RF signal. By using an AWG grating with m channels divided over theoptical bandwidth of the laser source the sampling rate requirements forthe A/D converters 610 is only the fraction of m of the overall systemsampling rate. The data of all A/D converters is combined in the memory611.

The frequency and time standards provided by frequency comb systems canfurther be distributed over long distances via fiber opticaltransmission lines. FIG. 7 a) shows such an embodiment of a fiber optictiming and wavelength distribution system comprising an optical clock701, as described with respect to FIG. 4, which feeds an optical combsignal into the distribution system, sections of stabilized fiber links704,706, signal recovery repeaters 705 and a photodetector 707. Ingeneral the timing distribution system contains multiple sections ofstabilized fiber links and signal recovery repeaters. The distributionsystem may also contain optical fiber amplifiers between stabilizedfiber links. It is also possible to have branches, eg. one signalrecovery repeater or optical amplifier can feed multiple stabilizedfiber links. The purpose of a fiber optic timing distribution system isto transmit the timing and wavelength information encoded in thefrequency comb signal at the optical input 702 to a remote location viaoutput port 708 with little deterioration in timing and wavelengthinformation. The optical input port 702 can be for example connected tooptical output port 408 of the optical clock shown in FIG. 4. Theoptical input signal fed into port 702 contains timing information whichcan be extracted by detecting the intermode beat signal of the frequencycomb lines as well as wavelength information realized by the individualcomb lines.

The optical input signal can be bandpass filtered by filter 703 toreduce the optical bandwidth and consequently to reduce pulse broadeningdue to dispersion in the stabilized fiber links. The timing informationcan be extracted via a low phase noise RF signal at the output port 709by detecting the intermode beat with detector 707. In another embodimentonly the timing information is transmitted in the distribution systemand the wavelength information is discarded at the signal recoveryrepeaters.

The maximum length of the stabilized fiber link sections is limited aswill be shown below. The implementation of signal recovery repeatersallows expansion of the total length of the fiber optic timing andwavelength distribution system beyond the maximum length of a singlestabilized fiber link.

FIG. 7 b) shows an embodiment of a stabilized optical fiber link. Otherimplementations of stabilized fiber links are for example disclosed inK. W. Holman, D. D. Hudson, J. Ye, and D. J. Jones, “Remote transfer ofa high-stability and ultralow-jitter timing signal,” Opt. Lett. 30,1225-1227. (2005). The embodiment shown in FIG. 7 b) comprises an inputport 720, a timing jitter or phase noise detector system 726, a feedbackloop 727, an optical delay line 728, a transmission fiber 729, a partialreflector 730, dispersion compensating elements 732 and opticalisolators at input and output ends 721 and 731. The feedback loop 727controls the optical delay in delay line 728 in such a way that theoptical path length of the optical transmission fiber 729 is constant.The error signal is generated by reflecting back part of the transmittedsignal by the partial reflector 730, coupling it out via fiber coupler725 at the input end and comparing it with part of the input signal 724.The back-reflected signal can be optically amplified by the opticalamplifier 723. The timing jitter detector 726 can be realized by anoptical cross correlator or an electronic phase noise sensor comprisingphotodetection of the forward and back-reflected light at ports 724 and722, RF amplifiers, phase shifters and mixers. Interferometric detectorssensing the optical phase difference between port 722 and 724 are alsopossible. The detector 726 measures the error signal caused by opticalpath length fluctuations of the transmission fiber 729. Since the signalat port 724 double-passes the fiber 729 the detector detectsapproximately twice the path length error of the single-pass transmittedlight over the transmission fiber 729 and the control signal has to becorrected for this fact by dividing the error signal by two, ensuringoptical path length stabilization for single-pass light through thetransmission fiber 729. In another embodiment the back-reflected lightfrom partial reflector 730 is back-transmitted in a different opticalfiber exposed to approximately the same environmental conditions as theforward transmitting fiber 729. For example both fibers can be part ofone fiberoptical cable. In this case the “backward fiber” can bedirectly connected to detector 726 and only a small part of the forwardtransmitted light needs to be coupled out at fiber coupler 725. In thisembodiment unidirectional fiberoptic amplifiers can be inserted in theforward transmission fiber 729 and the backward transmitting fiber. Inanother embodiment an optical circulator can be used to separate forwardand backward transmitted light, replacing the fiber coupler 725. In yetanother embodiment, a Faraday rotator can be inserted immediately beforethe partial reflector 730. This has the advantage that polarization modedispersion of the fiber 729 is compensated for the double pass lightused for error signal generation and the coupler 725 can be replaced bya polarizing beam splitter. The fiber port 722 may include dispersioncompensating elements to compensate the dispersive pulse broadening ofthe back-reflected light. Also dispersion compensating elements 723(e.g., dispersion compensating fiber, grating compressors, fiber Bragggrating compressors) may be inserted. Part or all of the dispersion offiber 729 can also be pre-compensated at the input port or at anyintermediate point of the transmission line. The delay line 728 canconsist of free space delay, fiberoptic stretchers, heated fibers orphase modulators. The partial reflector 730 can consist ofFresnel-reflectors at the fiber end, dielectric coatings, fiber Bragggratings or fiber loop mirrors. In the case of fiber Bragg gratings,these can be chirped in order to compensate dispersion for theback-reflected light.

Because the optical signal used for error signal generation needs todouble pass the transmission line, the most rapid optical path lengthfluctuations which can be compensated corresponds to the round trip timeof the optical signal 2*n*L/c, where n is the group index of thetransmission fiber, L the transmission fiber length and c the speed oflight. This relation limits the practical length of the transmissionfiber. To extend this length, signal recovery repeaters or opticalamplifiers are inserted between sections of stabilized fiber links inthe timing and wavelength distribution system.

FIG. 7 c) shows an embodiment of a signal recovery repeater,regenerating a phase coherent copy of the input optical frequency comb750 at the output port 765. Previously, the phase coherent stabilizationof two femtosecond frequency comb lasers has been disclosed; for examplein T. R. Schibli, J. Kim, O. Kuzucu, J. T. Gopinath, S. N. Tandon, G. S.Petrich, L. A. Kolodziejski, J. G. Fujimoto, E. P. Ippen, and F. X.Kaertner, “Attosecond active synchronization of passively mode-lockedlasers by balanced cross correlation,” Opt. Lett. 28, 947-949 (2003), orin A. Bartels, N. R. Newbury, I. Thomann, L. Hollberg, and S. A.Diddams, “Broadband phase-coherent optical frequency synthesis withactively linked Ti:sapphire and Cr:forsterite femtosecond lasers,” Opt.Lett. 29, 403-405. (2004). However none of the prior art phasecoherently stabilizes a frequency comb laser to a fiber-transmittedoptical frequency comb.

The signal recovery repeater comprises a frequency comb laser 762,detectors for repetition rate stabilization 754, CEO frequencystabilization 755 and feedback loops 761 and 757 for repetition rate andCEO stabilization. Part of the optical input signal from port 750 andpart of the optical output signal of the frequency comb laser 762 areconnected to repetition rate detector 754. The repetition rate detectorcan consist of an electronic phase locked loop comprisingphotodetectors, RF-amplifiers, RF bandpass filters, phase detectors andloop filters, or an optical cross-correlator. Also a combination ofelectronic and optical detection can be of advantage, since theelectronic detector has a larger capture range and the optical crosscorrelator has higher phase sensitivity. The electronic detector can beused for first coarse locking and slow drift stabilization inconjunction with an optical cross correlator driving a fast actuator.Once the repetition rate or equivalently the comb spacing of thefrequency comb laser 762 is stabilized to the input frequency comb viathe detector 754 and the feedback loop 761, the CEO frequency of laser762 can be stabilized to the input frequency comb 750. This is done bycombining part of the light of the input frequency comb with part of thelight from the frequency comb laser 762 in a fiber coupler 756 anddetecting a RF beat signal at a photodiode 755. Since the mode spacingof the input frequency comb and the frequency comb laser 762 is alreadyequal, the intermode beat signal of both frequency combs overlaps. Inaddition to the intermode beat signal, a beat signal corresponding tothe difference of the CEO frequency of input frequency comb 750 andfrequency comb laser 762 is detected. Using this stabilization scheme,neither an octave spanning bandwidth nor an f-2f interferometer isrequired for the frequency comb laser 762. Laser 762 can be realized bya mode-locked fiber oscillator with repetition rate and CEO phasecontrol inputs. The beat signal related to the CEO frequency differencecan be stabilized by feedback controlling the frequency comb laser 762via the control loop 760 to an offset frequency, most conveniently afraction of the intermode beat signal. The frequency of the optical comblines of frequency comb laser 762 will then be shifted by this offset.This shift can be alternatively compensated by frequency shifting one ofthe input ports of coupler 756 by the same amount with an acoustooptical modulator.

Alternatively, if wavelength transmission is not required, the CEOoffset detection and stabilization can be omitted. The wavelengthinformation can be regenerated from the timing information at the end ofthe transmission line using a frequency comb laser.

One part of the optical output of frequency comb laser 762 istransmitted to port 763, optionally bandpass filtered with filter 764and transmitted to the output port of the signal recovery repeater 765.This signal can now be transmitted in a subsequent stabilized fiberlink.

Finally, in addition to optical A/D converters, the fiber lasersdescribed above are also ideal for the construction of optical samplingunits required for the measurement of high bit rate signal sources asused in telecommunications. The implementation of a generic opticalsampling unit 800 is shown in FIG. 8. The sampling source 801 representsa high repetition rate pulse source as described in the '302publication. When using Bi—Er fiber, the sampling source 801 can beoperated at wavelengths in the range from 1500-1530 nm. The samplingsource is typically operated at a frequency f/n+δf, which is slightlyoffset from an integer fraction of the frequency f of the signal source802 to be sampled. Signal source 802 can for example be a repetitivetrain of pulses at a repetition rate of 40 GHz. The sampling frequencyof f/n+δf is derived from signal source 802 and generated in anelectronic mixer and frequency shifter 803; the sampling frequency isthen used to control the repetition rate of sampling source 801, whichis electronically locked to the sampling frequency f/n+δf.Alternatively, no electronic locking between signal source 702 andsampling source 801 is required to sample the data train from signalsource 802. The optical output 804 from signal source 802 is combinedwith the output 805 from sampling source 801 in a wavelength divisionmultiplexing coupler 806. For example the signal pulse train can be at awavelength of 1560 nm and the sampling pulse train can be at awavelength of 1520 nm. Four wave-mixing in a length of highly nonlinearfiber 807 is then used to generate a data output at a wavelength near1480 nm. The data output contains the information of the sampled signalpulse train at the reduced frequency of δf. The data output originatesfrom the nonlinear interaction between the signal pulse train at 1560 nmand the sampling pulse train at 1520 nm. The data output is convenientlyfiltered out by filter 808 and detected by detector 809. The output fromdetector 809 can then be digitized and processed to represent the signalpulse train using conventional techniques well known in the state of theart. The advantage of Bi-fiber based sampling sources is that signalsources in the whole spectral range of the central band oftelecommunications from 1530-1570 nm can be sampled with a Bi-fiberoperating at wavelengths<1530 nm or at wavelengths>1570 nm. Theseextreme wavelengths are not easily accessible with standard modelockedsources based on silica fibers.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. It is sought, therefore, to cover allsuch changes and modifications as fall within the spirit and scope ofthe invention, as defined by the appended claims, and equivalentsthereof.

1. A laser system comprising a modelocked fiber laser producing pulseswith a peak pulse power, said laser system configured to provide anoutput of equidistant optical pulses at a repetition frequency, saidlaser system comprising a phase adjuster controlling the absolute phaseof said optical pulses, said repetition frequency and said pulse phasefurther being matched to an external enhancement cavity disposedexternal to said modelocked fiber laser, such that the pulse peak powerwithin the boundaries of said external enhancement cavity issubstantially higher compared to the pulse peak power provided by saidmodelocked fiber laser.
 2. The laser system according to claim 1,wherein said pulses produced with said modelocked fiber laser areamplified in a chirped pulse amplification system.
 3. The laser systemaccording to claim 2, wherein said laser system generates at least oneof VUV, EUV and THz radiation in said enhancement cavity via theinsertion of a nonlinear material into said enhancement cavity.
 4. Thelaser system according to claim 2, wherein a nonlinear material isdisposed in said enhancement cavity, said non-linear material comprisinga gas, a liquid or a solid.
 5. The laser system according to claim 4,where said nonlinear material is a portion of a parametric amplifier. 6.The laser system according to claim 1, wherein said laser system isconfigured as a portion of a fiber-based frequency comb source, andwherein said absolute phase control is adjustable to produce anindividual laser comb linewidth which is smaller than the linewidth ofthe individual comb lines comprising the frequency comb spectrum of theenhancement cavity, and wherein the fiber-based frequency comb isadjusted to overlap with the frequency comb spectrum of the enhancementcavity, and wherein the comb spacing of the fiber-based comb spectrum isadjusted to be equal to the comb spacing of the comb spectrum of theenhancement cavity.
 7. A source of phase stable optical pulses,comprising: a modelocked fiber laser; a fiber amplifier to amplify anoutput of said modelocked fiber laser; and a phase stabilizer configuredsuch that a stable carrier phase slip is obtainable at the output ofsaid fiber amplifier.
 8. A coherent fiber-based frequency comb source,comprising: a passively modelocked fiber laser comprising an opticalcavity having intra-cavity components and a net cavity dispersion,wherein said cavity is configured in such a way that the net cavitydispersion is substantially lower than the dispersion of anyintra-cavity component; a fiber amplifier receiving outputs from saidpassively modelocked fiber laser and generating amplified output pulses;a highly nonlinear waveguide based on a crystalline material, saidwaveguide receiving amplified output pulses from said fiber amplifierand generating a coherent super-continuum therewith; an f-2finterferometer to measure the carrier envelope offset frequency of saidfiber comb source.
 9. The coherent fiber-based frequency comb source ofclaim 8, wherein said passively modelocked fiber laser generates outputsat a repetition rate greater than 110 MHz.
 10. The coherent fiber-basedfrequency comb source of claim 8, further comprising: a phase stabilizercomprising said f-2f interferometer; an adjustable control elementwithin or coupled to said passively modelocked fiber laser; and afeedback loop connecting said adjustable control element and said f-2finterferometer, said stabilizer being configured to stabilize thecarrier envelope offset frequency of said fiber comb source.
 11. Thecoherent fiber-based frequency comb source of claim 8, wherein saidnon-linear waveguide comprises a poled crystalline material.
 12. Acoherent fiber-based frequency comb source, comprising: a passivelymodelocked fiber laser; a fiber amplifier receiving outputs from saidpassively modelocked fiber laser and generating amplified output pulses;a highly nonlinear waveguide based on a poled crystalline material, saidwaveguide receiving amplified output pulses from said fiber amplifierand generating a super-continuum therewith, wherein said poledcrystalline material is periodically poled with a variation of thepoling period along the crystal length.
 13. The fiber-based frequencycomb source of claim 12, wherein a poling period of said poledcrystalline material is configured to produce at least one region ofenhanced spectral density within said super continuum.
 14. Thefiber-based frequency comb source of claim 12, wherein said periodicallypoled crystalline material comprises LiNbO₃.
 15. A coherent fiber-basedfrequency comb source, comprising: a passively modelocked fiber lasergenerating outputs at a repetition rate greater than 110 MHz, saidpassively mode locked fiber laser comprising an optical cavity havingintra-cavity components and a net cavity dispersion, wherein said cavityis configured in such a way that the net cavity dispersion issubstantially lower than the dispersion of any intra-cavity component; afiber amplifier receiving outputs from said passively modelocked fiberlaser and generating amplified output pulses; a highly nonlinearwaveguide, said waveguide receiving amplified output pulses from saidfiber amplifier and generating a coherent super-continuum therewith; anda stabilizer for a carrier envelope offset frequency of said fiber combsource.
 16. The coherent fiber-based frequency comb source of claim 15,wherein said stabilizer comprises at least one external referencecavity, and wherein at least two feedback loops are configured to alignsaid carrier envelope offset frequency and the repetition rate of saidpassively mode locked fiber laser to said external reference cavity. 17.The coherent fiber-based frequency comb source of claim 15, wherein saidstabilizer comprises at least two feedback loops configured withdetectors for receiving two narrow spectral portions of a spectraloutput of said comb source.
 18. The coherent fiber-based frequency combsource of claim 15, wherein said passively modelocked fiber laser isdiode pumped with at least one pump diode, and said stabilizer comprisesa feedback system to regulate the power of said at least one pump diode.19. The coherent fiber-based frequency comb source of claim 15, whereinsaid passively modelocked fiber laser comprises a fiber Bragg grating,and said stabilizer comprises a feedback system to regulate at least oneof temperature and stress of said fiber Bragg grating.
 20. The coherentfiber-based frequency comb source of claim 15, wherein said stabilizeris configured to utilize said continuum.
 21. The coherent fiber-basedfrequency comb source of claim 15, wherein said stabilizer comprises afeedback loop controlling said passively modelocked fiber laser based ona control signal derived from an output of said highly nonlinearwaveguide.